My DH, DS and I have annual passes to Disneyland and try to go often. I have a lot of really bad parenting there. On New Years, in the space of about 3 minutes, I saw two sad cases. This couple had their son on a very dirty leash. The kid looked a little too old to be on one (I've personally never used one, but to each their own). Like all kids at a theme park, this one was trying to climb up on the fence. It was low to the ground and concrete. What does the dad do? He just yanks him backwards. The kid falls onto the concrete ground and the parents just scold him and stand him up.
In front of them was another family. The son was caught up looking at everything and missed that the line moved. The dad grabbed him by the collar and jerked him up there, scolding him and then he slapped him in the back of the head.
The worst I've seen though has to be the "mom" who was screaming at her handicapped son in his wheelchair while he balled his eyes out. It looked like he had dropped his ice cream. That was the most heartbreaking thing I've seen there I think.
Have you seen some really bad parenting in public? It's really hard for me not to say or do anything in those situations.